1. Field of the Invention
The present invention relates to a demagnetizer and, more particularly, to a device capable of realizing an auto-degaussing operation in a cathode-ray tube (CRT).
2. Description of the Prior Art
An auto-degaussing operation is performed by causing flow of an attenuation AC exciting current in a degauss coil on the surface of a cathode-ray tube so as to eliminate any harmful influence of the terrestrial magnetism exerted on a scanning electron beam in the tube. And there is known an exemplary degaussing circuit employing a thermistor of positive characteristic.
Such conventional example has a demagnetizing circuit 1 as shown in FIG. 2, wherein an output of an AC power source 2 (normally a commercial power source) is supplied via a main switch 3 and a thermistor 4 of positive characteristic to a degauss coil 5.
The thermistor 4 has such positive characteristic as graphically represented by a curve K1 in FIG. 3, wherein the resistance of the thermistor 4 increases in accordance with temperature rise due to the self-heating phenomenon. Utilizing such characteristic, the degauss exciting current i flowing in the degauss coil 5 is so controlled that the peak value thereof is changed toward a zero level in accordance with the lapse of time as graphically shown in FIG. 4, whereby the electron-beam scanning space in the cathode-ray tube is magnetized by the degauss magnetic field generated from the degauss coil 5, in such a manner as to be retained free from any harmful influence of the terrestrial magnetism.
In the above structure, the positive-characteristic thermistor 4 is normally heated by a heating resistor 6 and is thereby held in a high resistance state.
When the demagnetizing circuit 1 of the constitution shown in FIG. 2 is employed in a demagnetizer with a commercial power supply used as the AC power source 2, if the supply voltage is different depending on the power distributed area, there arises a problem that, since the degauss exciting current i flowing in the degauss coil 5 is determined by the supply voltage of the AC power source 2, the intensity of the degauss magnetic field generated in the degauss coil 5 is varied due to such voltage difference, hence inducing a possibility of some unexpected harmful influence.
For example, the commercial supply voltages are 100 volts AC in Japan, 120 volts AC in U.S.A., 220 volts AC in the European countries, and 240 volts AC in U.K., respectively.
If any commercial power supply of a different voltage is used directly as the AC power source 2, then the intensity of the degauss magnetic field generated in the degauss coil 5 is rendered excessively great in case the supply voltage is higher, thereby raising some problems including undesired demagnetization of "transfer magnetic field" in the cathode-ray tube, and erroneous demagnetization of recorded information on a magnetic disk due to leakage of the degauss magnetic field to the outside when the disk is placed on the casing of a display unit or the like which incorporates the cathode-ray tube therein.
In the process of manufacture, the entire cathode-ray tube is intensely magnetized by the transfer magnetic field for rectifying the landing pattern of an electron beam in the tube, so that a mislanding pattern in the shape of, for example, a Japanese character " " or "inverted " can be corrected to consequently improve the yield rate in producing the cathode-ray tube.
In one of the methods adopted for solving such a problem, there are prepared a plurality of thermistor circuits having mutually different positive characteristics as represented by curves K21, K22 and so forth in FIG. 3 correspondingly to a plurality of predetermined supply voltages usable as the AC power source 2, and such thermistor circuits are selectively switched in conformity with the individual supply voltages so that the selected one is connected to the degauss coil 5. However, the device constitution as a whole is complicated due to the necessity of providing a plurality of such positive-characteristic thermistor circuits.